Enhancing CO2 absorption efficiency using a novel PTFE hollow fiber membrane contactor at elevated pressure

The internal structure design of membrane module is very important for gas removal performance using membrane contactor via physical absorption. In this study, a novel membrane contactor developed by weaving polytetrafluoroethylene (PTFE) hollow fibers was applied to remove CO₂ from 60% N₂ + 40% CO₂ mixture (with CO₂ concentration similar to that of biogas) at elevated pressure (0.8 MPa) using water as absorbent. Compared with the conventional module with randomly packed straight fibers, the module with woven PTFE fibers exhibited much better CO₂ absorption performance. The weaving configuration facilitated the meandering flow or Dean vortices and renewing speed of water around hollow fibers. Meanwhile, the undesired influences such as channeling and bypassing were also eliminated. Consequently, the mass transfer of liquid phase was greatly improved and the CO₂ removal efficiency was significantly enhanced. The effects of operation pressure, module arrangement, feed gas, and water flow rate on CO₂ removal were systematically investigated as well. The overall mass‐transfer coefficient (K_OV) varied from 1.96 × 10^?5 to 4.39 × 10^?5 m/s (the volumetric mass‐transfer coefficient K_La = 0.034–0.075 s^?1) under the experimental conditions. The CO₂ removal performance of novel woven fiber membrane contactor matched well with the simulation results. © 2017 American Institute of Chemical Engineers AIChE J, 64: 2135–2145, 2018     

Reduction of CO2 emissions by a membrane contacting process☆

A membrane-based gas–liquid contacting process was evaluated in this work for CO₂ removal from flue gases. The absorption of CO₂ from a CO₂–N₂ mixture was investigated using a commercial hollow fiber membrane contactor and water or diethanolamine as absorbing solvents. Significant CO₂ removal (up to 75%) was achieved even with the use of pure water as absorbent. By using aqueous amine solutions and chemical absorption, mass transfer improved, and CO₂ removal was nearly complete (∼99%). A mathematical model was developed to simulate the process and it was validated with experimental data. Results show that membrane contactors are significantly more efficient and compact than conventional absorption towers for acid gas removal.     

New absorption liquids for the removal of CO2 from dilute gas streams using membrane contactors

A new absorption liquid based on amino acid salts has been studied for CO₂ removal in membrane gas-liquid contactors. Unlike conventional gas treating solvents like aqueous alkanolamines solutions, the new absorption liquid does not wet polyolefin microporous membranes. The wetting characteristics of aqueous alkanolamines and amino acid salt solutions for a hydrophobic membrane was studied by measuring the surface tension of the liquid and the breakthrough pressure of the liquid into the pores of the membrane. The dependence of the breakthrough pressure on surface tension follows the Laplace-Young equation. The performance of the new absorption liquid in the removal of CO₂ was studied in a single fiber membrane contactor over a wide range of partial pressures of CO₂ in the gas phase and amino acid salt concentrations in the liquid. A numerical model to describe the mass transfer accompanied by multiple chemical reactions occurring during the absorption of CO₂ in the liquid flowing through the hollow fiber was developed. The numerical model gives a good prediction of the CO₂ absorption flux across the membrane for the absorption of CO₂ in the aqueous amino acid salt solutions flowing through the hollow fiber.     

离子液体-水复配吸收剂捕获CO2性能

基于绿色合成方法制备出亲水性离子液体(ILs)[NH₂-C₃mim][Br],从有效降低CO₂吸收-解吸操作成本出发,采用ILs-H₂O复配吸收剂,开展了常温加压CO₂吸收及吸收剂常温减压解吸再生实验。结果表明,比CO₂吸收量(基于复配吸收剂或离子液体组分)随复配吸收剂中ILs组分浓度而变;吸收初期,CO₂吸收速率随吸收剂配比变化显著;以CO₂高吸收率和吸收剂低成本为目标,优选出新型水基复配吸收剂(离子液体与水质量比为1.38:1)。分别以水基离子溶液、改良热钾碱液和活化复配醇胺液为吸收剂,在自行搭建的超重力场强化吸收-连续逆流接触(加热或减压)解吸再生台架实验装置上进行了CO₂捕获与吸收剂再生连续化实验。结果表明,在超重力场作用下,改良热钾碱液和活化复配醇胺液对CO₂有较好的捕获,吸收率分别在98%、96%和90%以上,3种吸收剂经加热或减压解吸再生后均可循环回用,水基离子溶液吸收剂在常温减压下解吸更具有实际可操作性。     

CO2 absorption into K2CO3/KHCO3 solution enhanced by organic phase in a rotor–stator reactor

ABSTRACT

This work studied the gas–liquid–liquid (G–L–L) reaction system of CO₂ absorption into K₂CO₃/KHCO₃ buffer solution enhanced by organic phase in a rotor–stator reactor (RSR). The effects of volume fraction of organic phase, type of organic phase, rotational speed of RSR, gas and liquid volumetric flow rate, and temperature on CO₂ absorption percentage were investigated. Results indicate that the addition of the organic phase with a volume fraction of 1.3–1.6% had significant promoting effect on CO₂ absorption. CO₂ absorption percentage increased with increase in the rotational speed of the RSR but decreased with rise in liquid temperature and gas–liquid ratio. This work demonstrates that RSR can significantly enhance liquid–liquid mixing and gas–liquid mass transfer processes in the G–L–L system.     

Modeling and experimental studies on absorption of CO2 by Benfield solution in rotating packed bed

This work presents the modeling and experimental investigation on absorption of CO₂ by Benfield solution in rotating packed bed (RPB). A model was established to illustrate the mechanism of gas–liquid mass transfer with reactions in RPB at higher gravity level. Experiments were carried out at various rotating speeds, liquid flow rates, gas flow rates and temperatures in RPB, with Benfield solution as the absorbent. The validity of this model was demonstrated by the fact that most of the predicted y_o (mole fraction of CO₂ in outlet gas) agreed well with the experimental data with a deviation within 10%. The presented profile of K_Ga (gas-phase volumetric mass transfer coefficient) along the radial direction of the packing could reasonably explain the end effect in RPB. As a result, this model is reliable in describing the removal of CO₂ by Benfield solution in RPB at higher gravity level.     

PVDF中空纤维膜接触器分离烟气CO2

以水（H2O）、氢氧化钠（NaOH）、氨基乙酸钾（GLY）、氨基乙酸钾-哌嗪（GLY-PZ）水溶液为吸收剂,研究了疏水性聚偏氟乙烯（PVDF）中空纤维膜接触器分离CO2/N2模拟烟气中CO2的技术,具体考察了流动方式、气液流率、吸收液浓度和温度、原料气CO2浓度、填充密度等对膜接触器吸收效率的影响。结果表明,气液逆流的腔流程模式具有较高的分离效率。不同吸收剂的分离性能为：NaOH > GLY-PZ > GLY > H2O。温度对各种吸收剂的影响随其种类不同而有所差异。膜接触器对烟气CO2的分离效率随填充密度、吸收液浓度和流率的提高而增大,随气体流率及其中CO2浓度的增大而减小。     

Mass transfer performance and correlations for CO2 absorption into aqueous blended of DEEA/MEA in a random packed column

The mass transfer performance of CO₂ absorption into blended N,N‐diethylethanolamine (DEEA)/ethanolamine (MEA) solutions was investigated using a lab‐scale absorber (H = 1.28 m, D = 28 mm) packed with Dixon ring random packing. The mass transfer coefficient K_Ga_v, the unit volume absorption rate Φ, outlet concentration of CO₂ (y_CO2), and the bottom temperature T_bot of CO₂ in aqueous DEEA/MEA solutions were determined over the feed temperature range of 298.15–323.15 K, lean CO₂ loading of 0.15–0.31 mol/mol, over a wide range of liquid flow rate of 3.90–9.75 m³/m²‐h, by using inert gas flow rate of 26.11–39.17 kmol/m²‐h and 6–18 kPa CO₂ partial pressure. The results show that liquid feed temperature, lean CO₂ loading, liquid flow rate, and CO₂ partial pressure had significant effect on those parameters. However, the inert gas flow rate had little effect. To allow the mass transfer data to be really utilized, K_Ga_v and y_out correlations for the prediction of mass transfer performance were proposed and discussed. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3048–3057, 2017     

Efficient capture of carbon dioxide with novel mass‐transfer intensification device using ionic liquids

A novel mass‐transfer intensified approach for CO₂ capture with ionic liquids (ILs) using rotating packed bed (RPB) reactor was presented. This new approach combined the advantages of RPB as a high mass‐transfer intensification device for viscous system and IL as a novel, environmentally benign CO₂ capture media with high thermal stability and extremely low volatility. Amino‐functionalized IL (2‐hydroxyethyl)‐trimethyl‐ammonium (S)?2‐pyrrolidinecarboxylic acid salt ([Choline][Pro]) was synthesized to perform experimental examination of CO₂ capture by chemical absorption. In RPB, it took only 0.2 s to reach 0.2 mol CO₂/mol IL at 293 K, indicating that RPB was kinetically favorable to absorption of CO₂ in IL because of its efficient mass‐transfer intensification. The effects of operation parameters on CO₂ removal efficiency and IL absorbent capacity were studied. In addition, a model based on penetration theory was proposed to explore the mechanism of gas–liquid mass transfer of ILs system in RPB. © 2013 American Institute of Chemical Engineers AIChE J, 59: 2957–2965, 2013     

CO2 Adsorption Kinetics of K2CO3/Activated Carbon for Low‐Concentration CO2 Removal from Confined Spaces

K₂CO₃ supported on activated carbon (K₂CO₃/AC) is a promising means to remove low‐concentration CO₂ from confined spaces. In this removal process, physical adsorption plays an important role but it is difficult to quantify the amount of CO₂ adsorbed when both H₂O and CO₂ are present. The linear driving force mass transfer model is adopted to study the CO₂ adsorption kinetic characteristics of K₂CO₃/AC by analyzing the experimental data. The effect of K₂CO₃ and H₂O on the adsorption of CO₂ in K₂CO₃/AC was also evaluated. K₂CO₃ loaded on the support is found to increase the mass transfer resistance but decrease the activation energy required for the physical adsorption process. The presence of water vapor is disadvantageous to achieve high physical adsorption capacity since it enhances the chemical sorption in the competitive dynamic sorption process.     

Mathematical Modeling,Simulation, and Experimental Verification of CO2 Removal in a Turbulent Contact Absorber

A highly efficient technique of contaminant gas reduction, Turbulent Contact Absorber (TCA), is applied to CO₂ removal from a typical flue gas. Aqueous K₂CO₃ sorbent was evaluated as a regenerable sorbent for CO₂ from the flue gas. In order to identify the system, momentum and mass balance equations were written for the TCA tower. A flat plate falling film model was employed to simulate the TCA tower and the effect of turbulence was included in mass and momentum transfer coefficients. To check the accuracy of the model, a pilot scale TCA was built and operated. A Testo type gas analyzer was used to detect gas concentrations at the inlet and outlet of the rig. The model was validated successfully with pilot plant data. The effect of velocity and K₂CO₃ concentration on the TCA performance has also been carried out. It was found that the bed pressure drop increases linearly with gas velocity and then remains constant. An increase in the liquid flow rate increases liquid holdup, which leads to a rise in bed pressure drop. Higher turbulence within the TCA causes a velocity peak to shift from hypothetical gas‐liquid interface towards the falling film plate. An increase of the K₂CO₃ concentration from 1.0 g mol/L to 2.0 g mol/L was found to give an increase in CO₂ removal by about 4 %.     

Carbon dioxide absorption in a cross-flow rotating packed bed

This work investigates the feasibility of applying the cross-flow rotating packed bed (RPB) to the removal of carbon dioxide (CO₂) by absorption from gaseous streams. Monoethanolamine (MEA) aqueous solution was used as the model absorbent. Also, other absorbents such as the NaOH and 2-amino-2-methyl-1-propanol (AMP) aqueous solutions were compared with the MEA aqueous solution. The CO₂ removal efficiency was observed as functions of rotor speed, gas flow rate, liquid flow rate, MEA concentration, and CO₂ concentration. Experimental results indicated that the rotor speed positively affects the CO₂ removal efficiency. Our results further demonstrated that the CO₂ removal efficiency increased with the liquid flow rate and the MEA concentration; however, decreased with the gas flow rate and the CO₂ concentration. Additionally, the CO₂ removal efficiency for the MEA aqueous solution was superior to that for the NaOH and AMP aqueous solutions. Based on the performance comparison with the conventional packed bed and the countercurrent-flow RPB, the cross-flow RPB is an effective absorber for CO₂ absorption process.     

Absorption of carbon dioxide by the absorbent composed of piperazine and 2-amino-2-methyl-1-propanol in PVDF membrane contactor

This paper tests the performance of microporous polyvinylidinefluoride (PVDF) hollow fiber in a gas absorption membrane process (GAM) using the aqueous solutions of piperazine (PZ) and 2-amino-2-methyl-1-propanol (AMP). Experiments were conducted at various gas flow rates, liquid flow rates and absorbent concentrations. Experimental results showed that wetting ratio was about 0.036% when used with the aqueous alkanolamine solutions, while that was 0.39% with aqueous piperazine solutions. The CO₂ absorption rates increased with increasing both liquid and gas flow rates at N_Re < 20. The increase of the PZ concentration showed an increase of absorption rate of CO₂. The CO₂ absorption rate was much enhanced by the addition of PZ promoter. The resistance of membrane was predominated as using a low reactivity absorbent and can be neglected as using absorbent of AMP aqueous solution. The resistance of gas-film diffusion was dominated as using the mixed absorbents of AMP and PZ. An increase of PZ concentration, the resistance of liquid-film diffusion decreased but resistance of gas-film increased. Overall, GAM systems were shown to be an effective technology for absorbing CO₂ from simulated flue gas streams, but the viscosity and solvent-membrane relationship were critical factors that can significantly affect system performance.     

Gas absorption of carbon dioxide in a hollow fiber contained liquid membrane absorber

Experiments on the absorption of CO₂ into a hollow fiber contained liquid membrane absorber were performed. The feed gas was a mixture of CO₂ and N₂, absorbent liquid was 2-amino-2-methyl-l-propanol and the hollow fiber was a microporous hydrophobic polytetrafluoroethylene membrane. Outlet concentration of CO₂ from the absorber decreased as absorbent concentration increased, gas flow rate increased and were held constant for speed of agitation, but had a maximum value in the range of inlet concentration of CO₂ from 5 to 40 mole%. The reaction rate constant obtained for CO₂-amine system was 231 I/mol · s at 25 °C using a flat stirred vessel, and the membrane-side-mass-transfer coefficient was 1.217 × 10⁻⁵ mol/cm² · s · atm in CO2/N2-amine system. A diffusion model based on mass transfer with fast-reaction was proposed to predict the performance of the absorber.     

Absorption of carbon dioxide in aqueous MDEA solution coupling dust suppression under atomization

ABSTRACT

Large amounts of CO₂ and dust particles coming from power plant flue need to be captured and removed before flue is discharged into the air. In present work, absorption of carbon dioxide in aqueous N-methylidiethanolamine (MDEA) solution coupling dust suppression has been studied in an atomization absorption column, with MDEA concentrations ranging from 0.1 to 0.5mol/L, and with atomization frequencies ranging from 50 to 80 HZ. The obtained experimental results show that absorption rate of CO₂ in aqueous MDEA solution can be enhanced when the absorption process couples a dust suppression one under the condition of atomization. The reason for it is attributed to the adsorption of droplets on the solid particles which restrains the amount of entrainment and makes more droplets contact with gas so as to increase effective mass transfer area, thus resulting in the increase of CO₂ absorption rate. The range of obtained enhancement factor is from 1.1 to 1.7. Mass transfer enhancement factor increases with the increase of MDEA concentration and atomization frequency at a certain range. Effective mass transfer areas and entrainment ratios suppressed have been calculated based on theoretic research. The results calculated agree with our experimental phenomena, and support the enhancement mass transfer mechanism proposed.     

填料塔中碳酸钾/哌嗪混合吸收液脱除CO2的体积传质系数

The process of removing dilute CO2 from air by using me mixtures of K2CO3 and piperazine (PZ) was conducted in a random packed tower at 25℃. The results showed that PZ increased the absorption rate of CO2 into aqueous K2 CO3 much more effectively than MEA or DEA. The volumetric overall mass transfer coefficient (KGa) of dilute CO2 absorption into K2CO3/PZ was measured. The KGa value was evaluated over the ranges of main operating variables: the concentration of CO2 in inlet gas, gas flow rate, liquid loading, CO2 loading in liquid phase, and the concentrations of K2CO3 and PZ. The test showed that KGa could be remarkably improved by increasing liquid loading and the concentration of PZ, and decreasing the concentration of CO2 in inlet gas, as well as the gas flow rate and CO2 loading in liquid phase.     

Simultaneous desulfurization and denitrification of flue gas by pre-ozonation combined with ammonia absorption

A process of simultaneous desulfurization and denitrification of flue gas was conducted in this study. The flue gas containing 200 mg·m⁻³ NO, 1000–4000 mg·m⁻³ SO₂, 3%–9% O₂, and 10%–20% CO₂ was first oxidized by O₃ and then absorbed by ammonia in a bubbling reactor. Increasing the ammonia concentration or the SO₂ content in flue gas can promote the absorption of NO_X and extend the effective absorption time. On the contrary, both increasing the absorbent temperature or the O₂ content shorten the effective absorption time of NO_X. The change of solution pH had substantial influence on NO_X absorption. In the presence of CO₂, the NO_X removal efficiency reached 89.2% when the absorbent temperature was raised to 60 °C, and the effective absorption time can be maintained for 8 h, which attribute to the buffering effect in the absorbent. Besides, both the addition of Na₂S₂O₃ and urea can promote the NO_X removal efficiency when the absorbent temperature is 25 °C, and the addition of Na₂S₂O₃ had achieved better results. The advantage of adding Na₂S₂O₃ became less evident at higher absorbent temperature and coexistence of CO₂. In all experiments, SO₂ removal efficiency was always above 99%, and it was basically not affected by the above factors.     

Absorption of CO2 by aqueous diethanolamine (DEA) solutions in a high shear jet absorber

CO₂ diluted with N₂ was absorbed by aqueous DEA solutions in a high shear jet absorber consisting of a high pressure stainless steel vessel with a pressure nozzle at the top. The results show that equilibrium conditions can be rapidly reached under high shear conditions. CO₂ removal expressed by moles of CO₂ absorbed per mole of DEA increased with gas flow rate and decreased with liquid flow rate. The overall mass transfer capacity coefficient K_g a was found to increase with gas flow rate and solution concentration. K_g a went through a maximum with solution flow rate. Increasing the CO₂ partial pressure was found to decrease K_g a.     

Numerical simulation of the mass transfer process of CO2 absorption by different solutions in a microchannel

The flow and mass transfer characteristics of CO₂ absorption in different liquid phases in a microchannel were studied by numerical simulation. The mixture gas phase contained 5 vol% CO₂ and 95 vol% N₂ , and the different liquid phases were water, ethanol solution, 0.2 M monoethanolamine solution, and 0.2 M NaOH solution, respectively. Based on the permeation theory, the distribution of velocity and concentration in the slug flow was obtained by local simulation of flow and mass transfer coupling and was described in depth. The influence of contact time and bubble velocity on the mass transfer of the whole bubble was highlighted. The volumetric mass transfer coefficient on the bubble cap and liquid film, CO₂ absorption rate, and enhancement factor were calculated and analyzed. The results showed that the volumetric mass transfer coefficients of chemical absorption were ~3 to 10 times that of physical absorption and the CO₂ was absorbed more completely in chemical absorption. The new empirical correlations for predicting the mass transfer coefficient of the liquid phase were proposed respectively in physical absorption and chemical absorption, which were compared with the empirical formulas in the literature. The volumetric mass transfer coefficients obtained by predictive correlations are in good agreement with those obtained by simulation in this paper. This work made a basic prediction for CO₂ absorption in microchannel and provides a foundation for later experimental research.     

Characteristics of CO2 Absorption into Aqueous Ammonia

Abstract

Aqueous ammonia was investigated as a new absorbent of the chemical absorption process for CO₂ capture from combustion flue gas. The effects of the temperature and concentration of aqueous ammonia on CO₂ absorption in a semi‐batch reactor were studied by interpreting breakthrough curves. Raman spectroscopy analysis of CO₂ loaded aqueous ammonia provided concentration changes of bicarbonate, carbonate, and carbamate as well as CO₂ sorption capacity at given time during the absorption with 13 wt% aqueous ammonia at 25°C. It was observed that carbamate formation was dominating at the early stage of absorption. Then, the bicarbonate formation took over the domination at the later stage while the carbonate remained unchanged.